Syntheses, Structures and Properties of Two Cd(II) Complexes Based onthe 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazole Ligand

Xiao Su, Ting Li, Yu Xiu, and Xiangru Meng

The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou450052, P. R. China

Reprint requests to Prof. Xiangru Meng. Fax: +86-0371-67783126. E-mail: mxr@zzu.edu.cn

Z. Naturforsch. 2012, 67b, 678 – 684 / DOI: 10.5560/ZNB.2012-0083Received March 23, 2012

Two new 1-D Cd(II) complexes, {[Cd(imb)(µ2-Cl)Cl]·CH3OH}n (1) and {[Cd(imb)(µ2-Cl)Cl]·DMF}n (2), have been synthesized by the reactions of the unsymmetrical ligand 2-(1H-imidazol-1-yl-methyl)-1H-benzimidazole (imb) with CdCl2·2.5H2O in methanol or methanol/DMFsolution at room temperature, and structurally characterized by single-crystal X-ray diffraction. Com-plex 1 displays a ladder-shaped chain in which [Cd2Cl4] units are linked to each other through twobridging imb ligands. Complex 2 features an infinite looped chain structure composed of two kinds ofrings, the smaller [Cd2Cl2] ring and the larger [Cd2imb2] ring, connected alternately via their Cd(II)ions. The different architectures of complexes 1 and 2 can be generated by solvent exchange. The IRspectra and fluorescence properties of the complexes have been also investigated.

Key words: Cadmium Complex, 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazol, Crystal Structure,Fluorescence

Introduction

Metal-organic frameworks (MOFs) with intriguingstructures have received great attention in recent yearsdue to their interesting properties and potential ap-plications for electrical conductivity and magnetism,as sensors, in photocatalysis, and in porous materi-als [1 – 5]. The choice of the organic ligand is a keystep in the construction of specific architectures ofthe complexes because these are the organic units thatserve to coordinate to the metal centers and pass on thestructural information expressed in metal coordinationpreferences throughout the extended structure. In pur-suing various structures of complexes, N-heterocycliccompounds bearing flexible backbones like alkylspacers are one of the most widely studied classes ofligands because their flexibility and conformationalfreedom allow for greater structural diversity [6 – 8].In virtue of these advantages, a great number ofcomplexes based on flexible N-heterocyclic ligandshave been synthesized such as [Cd2(Hbimt)2I4],{[Cd(Hbimt)(SO4)(H2O)2]·1.5H2O}n (Hbimt = 2-((benzoimidazolyl)methyl)-1H-tetrazole), [Cd(L)0.5

(Hbtc)]·0.2H2O, [Zn3(L)(btc)2(H2O)2]·3H2O, (L =1,2,4,5-tetrakis(imidazol-1-yl-methyl)benzene, H3btc= 1,3,5-benzenetricarboxylic acid), [Cd5(N3)6(btz)2]n

(H2btz = 1,5-bis(5-tetrazolo)-3-oxapentane), [Cu2(imb)2Cl4]·2CH3OH, {[Cu(imb)(bdic2−)]·1.5H2O·DMF}n, and {[Cu(imb)(bdic2−)]·2H2O}n (imb = 2-(1H-imidazol-1-yl-methyl)-1H-benzimidazole, H2bdic = 1,3-benzenedicarboxylic acid) [9 – 12]. Fur-thermore, the choice of solvents also plays a criticalrole in the construction of specific metallosupramolec-ular architectures [13]. On the one hand, the solventoften influences the coordination behavior of themetal ions and then determines the constructionof the frameworks [14 – 16]. On the other hand,extra-framework solvent molecules can also influencethe structures of the complexes via intermolecularinteractions with the parent network [17, 18].

In this work, we used a flexible multidentate N-heterocyclic compound 2-(1H-imidazol-1-yl-methyl)-1H-benzimidazole (imb) as ligand to self-assemblewith Cd(II) salts in methanol or methanol/DMF,and obtained two new complexes {[Cd(imb)(µ2-Cl)Cl]·CH3OH}n (1) with ladder-shaped chains and

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X. Su et al. · Cd(II) Complexes Based on the 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazole Ligand 679

{[Cd(imb)(µ2-Cl)Cl]·DMF}n (2) with looped chains.Furthermore, the steady state fluorescence propertiesof the two complexes have been investigated.

Experimental Section

All chemicals were purchased of AR grade and used with-out further purification. The 2-(1H-imidazole-1-yl-methyl)-1H-benzimidazole (imb) was synthesized according to theliterature [19, 20]. The two complexes were characterized bytheir IR spectra on a Bruker Tensor 27 spectrophotometerwith KBr pellets in the range of 400 – 4000 cm−1. Elemen-tal analyses (C, H, and N) were performed on a Flash EA1112 elemental analyzer. Solid-state luminescence spectrawere recorded with a Fluoro Max-P fluorescence spectropho-tometer.

Synthesis of {[Cd(imb)(µ2-Cl)Cl]·CH3OH}n (1)

A methanol solution (3 mL) of imb was added drop-wise into a methanol solution (2 mL) of CdCl2·2.5H2O(0.1 mmol) to give a clear solution. Two weeks later col-orless crystals suitable for X-ray diffraction were obtaineddue to the slow evaporation of the solvent at room tempera-ture. Crystals of 1 are stable in the air. Yield: 47%. – Anal.for C12H14CdCl2N4O (413.57): calcd. C 34.82, H 3.39, N13.54; found C 34.88, H 3.47, N 13.66. – FT-IR (KBr, cm−1):

Table 1. Crystal and structure refinement data of complexes 1 and 2.

1 2Empirical formula C12H14CdCl2N4O C14H17CdCl2N5OFormula mass Mr 413.57 454.63Temperature, K 293(2) 293(2)Crystal size, mm3 0.21× 0.18× 0.15 0.19× 0.18× 0.15Crystal system triclinic triclinicSpace group P1 P1a, A 8.3263(17) 8.7941(18)b, A 10.048(2) 9.871(2)c, A 10.073(2) 11.704(2)α , deg 75.78(3) 71.15(3)β , deg 87.08(3) 86.38(3)γ , deg 73.75(3) 68.34(3)Volume, A3 784.1(3) 891.8(3)Z 2 2Calculated density, g cm−3 1.75 1.69Absorption coefficient, mm−1 1.7 1.5F(000), e 408 4522θ range for data collection, deg 5.26-55.72 3.68-55.76hkl range ±10,±13,−12→13 ±11,±12,±15Reflections collected / unique / Rint 8558 / 3686 / 0.0205 10902 / 4218 / 0.0279Data/restraints/parameters 3686 / 0 / 183 4218 / 0 / 208Final indicesR1 / wR2 [I > 2σ (I)] 0.0262 / 0.0593 0.0324 / 0.0920Final indices R1 / wR2 (all data) 0.0292 / 0.0613 0.0356 / 0.1039Goodness-of-fit on (F2) 1.035 1.010∆ρfin (max / min), e A−3 0.743 / −0.396 0.731 / −0.758

ν = 3429 (s), 3112 (m), 2947 (w), 1599 (m), 1517 (s), 1496(s), 1453 (s), 1421 (m), 1316 (m), 1280 (s), 1231 (s), 1087(s), 1007 (s), 842 (m), 745 (s), 658 (m).

Synthesis of {[Cd(imb)(µ2-Cl)Cl]·DMF}n (2)

A methanol solution (3 mL) of imb (0.1 mmol) was addeddropwise to a methanol solution containing CdCl2·2.5H2O(0.1 mmol). Then 1 mL DMF was added to the mixtureto give a clear solution. After two weeks at room tem-perature clear crystals were obtained. Yield: 53%. - Anal.for C14H17CdCl2N5O (454.63): calcd. C 36.99, H 3.77, N15.40; found C 37.03, H 3.63, N 15.44. – FT-IR (KBr, cm−1):ν = 3007 (m), 2361 (s), 2343 (w), 1686 (w), 1618 (s), 1560(m), 1438 (m), 1368 (s), 1277 (m), 1108 (m), 1027 (m), 937(w), 846 (w), 757 (m), 669 (w).

Single-crystal structure determination

The crystals were carefully selected and attached toa thin glass fiber. The data collections were performed ona Rigaku Saturn 724 CCD area detector with graphite-monochromatized MoKα radiation (λ = 0.71073 A; 50 kVand 40 mA) at a temperature of 293(2) K in ω scan modeand at a crystal-to-detector distance of 45 mm. The data werecorrected for Lorentz and polarization effects. An empiri-cal absorption correction was applied. The structures were

680 X. Su et al. · Cd(II) Complexes Based on the 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazole Ligand

Table 2. Selected bond lengths (A) and angles (deg) for complexes1 and 2a.

Complex 1 Complex 2Cd1–N3 2.2434(18) Cd1–N3 2.304(3)Cd1–N1#1 2.343(2) Cd1–N1#1 2.304(3)Cd1–Cl2 2.4684(12) Cd1–Cl2 2.4275(12)Cd1–Cl1 2.4993(9) Cd1–Cl1 2.5663(10)Cd1–Cl1#2 2.7589(10) Cd1–Cl1#2 2.6467(14)N3–Cd1–N1#1 89.56(7) N3–Cd1–N1#1 85.43(10)N3–Cd1–Cl2 112.26(5) N3–Cd1–Cl2 112.57(8)N1#1–Cd1–Cl2 93.01(6) N1#1–Cd1–Cl2 104.43(9)N3–Cd1–Cl1 119.56(6) N3–Cd1–Cl1 136.83(7)N1#1–Cd1–Cl1 90.83(6) N1#1–Cd1–Cl1 87.04(8)Cl2–Cd1–Cl1 128.05(3) Cl2–Cd1–Cl1 110.48(4)N3–Cd1–Cl1#2 89.32(5) N3–Cd1–Cl1#2 85.83(7)N1#1–Cd1–Cl1#2 175.73(5) N1#1–Cd1–Cl1#2 153.82(8)Cl2–Cd1–Cl1#2 91.23(4) Cl2–Cd1–Cl1#2 101.72(5)Cl1–Cd1–Cl1#2 86.14(3) Cl1–Cd1–Cl1#2 82.61(4)Cd1–Cl1–Cd1#2 93.86(3) Cd1–Cl1–Cd1#2 97.39(4)

a Symmetry transformations used to generate equivalent atoms: 1: #1 x + 1, y, z; #2 −x + 1, −y + 2, −z + 2; 2: #1 −x,−y+2, −z+1; #2 −x+1, −y+2, −z+1.

D–H· · ·A d(D–H) (A) d(H· · ·A) (A) d(D· · ·A) (A) (D–H· · ·A) (deg)

Complex 1N4–H4C · · ·O1 0.86 1.99 2.821(3) 162.1O1–H1 · · ·Cl2#4 0.82 2.37 3.161(2) 162.0

Complex 2N4–H4C · · ·O1 0.86 1.96 2.720(4) 146.7

a Symmetry transformation used to generate equivalent atoms: 1: #4 x, y−1, z.

Table 3. Hydrogen bonds ofcomplexes 1 and 2a.

solved by Direct Methods and refined by full-matrix least-squares and difference Fourier techniques, based on F2, us-ing the SHELXS/L-97 program package [21]. The hydrogenatoms were assigned common isotropic displacement param-eters and included in the final refinement by using geomet-rical restraints. Crystallographic parameters and structure re-finement data are summarized in detail in Table 1. Selectedbond lengths and bond angles are listed in Table 2. Hydrogenbond parameters are listed in Table 3.

CCDC 860559 and 860560 contain the supplementarycrystallographic data for this paper. These data can be ob-tained free of charge from The Cambridge CrystallographicData Centre via www.ccdc.cam.ac.uk/data request/cif.

Results and Discussion

IR spectroscopy of complexes 1 and 2

In the IR spectra the absorption band at 3429 cm−1

for 1 can be attributed to the stretching vibrations ofO-H. The absorption bands at 3112 cm−1 for 1 and at3007 cm−1 for 2 originate from stretching vibrations

of Ar-H. The sharp bands observed at 1517, 1496 and1453 cm−1 for 1, and at 1618, 1560 and 1438 cm−1 for2, are due to stretching vibrations of C=C and C=N. Inaddition, the sharp bands at 1618 cm−1 for 2 show theexistence of solvate DMF molecules [22]. The bands at1280 cm−1 for 1 and 1277 cm−1 for 2 are the results ofC-N stretching vibrations. The bands at 745 cm−1 for1 and 757 cm−1 for 2 belong to stretching vibrationsof the disubstituted phenyl ring. The above analysesare consistent with the results of the X-ray diffractionstudy.

Crystal structure of {[Cd(imb)(µ2-Cl)Cl]·CH3OH}n

(1)

Single-crystal X-ray diffraction analysis has re-vealed that complex 1 crystallizes in the triclinic spacegroup. As depicted in Fig. 1a, the Cd(II) ion is in a dis-torted trigonal-bipyramidal coordination environmentwith two nitrogen atoms from the benzimidazole andimidazole rings of two imb ligands, two bridging chlo-

X. Su et al. · Cd(II) Complexes Based on the 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazole Ligand 681

Fig. 1 (color online). (a) Coordination en-vironment of Cd(II) in 1 with ellip-soids drawn at the 30% probabilitylevel (hydrogen atoms and free methanolmolecules were omitted for clarity); (b)ladder chain of complex 1 parallel tothe crystallographic a direction (hydrogenatoms and methanol molecules are omit-ted for clarity); (c) 2-D structure of com-plex 1 constructed by hydrogen bonds andπ· · ·π stacking interactions.

ride anions and one terminal chloride anion. The equa-torial plane is occupied by N3, Cl1, Cl2, and Cd(II)atoms with the mean deviations of 0.0185 A, whilethe Cl1#2 and N1#1 atoms occupy the axial positionswith an N1#1–Cd1–Cl#2 angle of 175.73(5)◦. Cd1 andCd1#2 are bridged by two chloride atoms to give a di-nuclear unit [Cd2Cl4] with a Cd1· · ·Cd1#2 distance of3.846(1) A. Each [Cd2Cl4] unit is connected with twoidentical units through two bridging imb ligands, lead-ing to the formation of a ladder-shaped chain paral-lel to the a direction, as shown in Fig. 1b. The di-hedral angle between benzoimidazole and imidazole

rings in the imb ligand is 68.01(9)◦. The intrachainCd· · ·Cd distance is 8.3263 A. In complex 1, there areO–H· · ·Cl and N–H· · ·O hydrogen bonds between un-coordinated methanol molecules and terminal chlorineatoms (O· · ·Cl: 3.161(2) A), and between the benzoim-idazole rings and uncoordinated methanol molecules(N· · ·O: 2.821(3) A). The ladder-shaped chains are fur-ther connected into a 2-D structure through hydro-gen bonds and π· · ·π stacking interactions betweenbenzoimidazole rings of adjacent chains (the centroid-to-centroid distance is 3.6096(11) A), as shown inFig. 1c.

682 X. Su et al. · Cd(II) Complexes Based on the 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazole Ligand

Fig. 2 (color online). (a) Coordination en-vironment of Cd(II) in 2 with ellipsoidsdrawn at the 30% probability level (hy-drogen atoms and free DMF moleculeswere omitted for clarity); (b) looped chainstructure of complex 2 composed of twokinds of rings parallel to the crystallo-graphic a direction; (c) view of the π· · ·πstacking interactions between benzoimi-dazole rings in complex 2.

Crystal structure of {[Cd(imb)(µ2-Cl)Cl]·DMF}n (2)

The introduction of DMF into the reaction systeminfluences the final structure of the product and givesrise to a new complex 2 with a different structure ascompared to 1. In complex 2, the imb ligands coordi-nate to Cd(II) ions with the dihedral angle of the ben-zoimidazole and imidazole rings being 72.658(15)◦.Thus complex 2 displays an infinite looped chain struc-ture composed of two kinds of rings (Fig. 2b). Thesmaller ring is formed by two chloride anions bridg-ing two Cd(II) cations with a Cd· · ·Cd distance of3.917(1) A. The larger ring is defined by two imbmolecules bridging two Cd(II) ions, where the Cd· · ·Cddistance is 7.081(2) A. The two kinds of rings areconnected alternately via Cd(II) ions yielding an in-

finite chain. There are N–H· · ·O hydrogen bonds be-tween benzimidazole rings and uncoordinated DMFmolecules (N· · ·O: 2.720(4) A), and π· · ·π stackinginteractions between the benzimidazole rings of ad-jacent chains with a centroid-to-centroid distances of3.904(20) A (Fig. 2c). The coordination geometry ofthe Cd(II) ion is similar to that of complex 1, as shownin Fig. 2a. Each Cd(II) cation is five-coordinated bytwo nitrogen atoms from the benzimidazole and im-idazole rings of two imb ligands, two bridging chlo-ride anions and one terminal chloride anion in a dis-torted trigonal-bipyramidal coordination environment.The equatorial plane is completed by N3, Cl1, Cl2, andCd(II) with a mean deviation of 0.0176 A, while Cl1#2

and N1#1 occupy the axial positions with an N1#1–Cd1–Cl1#2 angle of 153.82(8)◦.

X. Su et al. · Cd(II) Complexes Based on the 2-(1H-Imidazol-1-yl-methyl)-1H-benzimidazole Ligand 683

Fluorescence spectra

A number of Zn(II) and Cd(II) complexes havebeen investigated regarding their fluorescence proper-ties because of their potential applications in chemi-cal sensors, photochemistry, and light-emitting diodes(LEDs) [10, 23, 24]. The solid-state luminescenceproperties of complexes 1 and 2 were investigated at

Fig. 3 (color online). Solid-state emission spectra of the imbligand and the complexes 1 and 2 at room temperature.

room temperature. As shown in Fig. 3, the main emis-sion peaks of the complexes are observed at 318 nmfor 1 when excited at 293 nm, and at 317 nm for 2when excited at 285 nm. The photoluminescence spec-trum of the free ligand imb was also measured in thesolid state. It displays an intense emission band at305 nm (λex = 285 nm) attributable to π → π∗ tran-sitions. The emission spectra of complexes 1 and 2are also tentatively assigned to the intra-ligand chargetransfer which is slightly red-shifted owing to the co-ordination of organic ligands to the metal ions [10].

Conclusion

In this work, we have prepared and characterizedtwo new chain frameworks with different structuresbased on imb ligands from CdCl2·2.5H2O in differentsolvent systems. The results indicate that the rationalchange of solvent can influence significant details ofthe architectures of the complexes.

Acknowledgement

We gratefully acknowledge the financial support bythe National Natural Science Foundation of China (no.J0830412).

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